The present invention relates to the arrangement of heat exchange media in regenerative thermal oxidizers (RTOs), and the resulting improved thermal oxidizers.
Regenerative thermal oxidizers are generally used for destroying volatile organic compounds (VOCs) in high flow, low concentration emissions from industrial and power plants. RTOs typically require high oxidation temperatures in order to achieve high VOC destruction and high heat recovery efficiency. To more efficiently attain these characteristics, the "dirty" process gas which is to be treated is preheated before oxidation in a heat exchanger column. The column is usually packed with a heat exchange material having good thermal and mechanical stability and high thermal mass. In operation, the process gas is fed through a previously heated heat exchanger column, which, in turn, heats the process gas to a temperature approaching or attaining its VOC oxidation temperature. This pre-heated process gas is then directed into a combustion zone where VOC oxidation is usually completed, if necessary. The treated "clean" gas is then directed out of the combustion zone and back through the heat exchanger column, or, according to a more efficient process, through a second heat exchange column. As the hot oxidized gas is fed through this second column, the gas transfers its heat to the heat exchange media in that column, cooling the gas and pre-heating the heat exchange media so that another batch of process gas may be preheated through the second column after the oxidizer is cycled. Usually, an RTO has at least two heat exchanger columns which alternately receive process and treated gases. This process is continuously carried out, allowing process gas to be efficiently treated.
The performance of an RTO may be optimized by increasing VOC destruction efficiency and by reducing operating and capital costs. The art of increasing VOC destruction efficiency has been addressed in the literature using, for example, means such as improved oxidation systems and purge systems. Operating costs can be reduced by increasing the heat recovery efficiency, and by reducing the pressure drop across the oxidizer. Operating and capital costs may be reduced by properly designing the RTO and by selecting appropriate heat transfer packing materials. While design aspects of RTOs have been the subject of prior patent literature, the arrangement of the heat transfer packing material has not been sufficiently addressed.
A horizontal arrangement for a heat exchanger may be used in applications such as a regenerative oxidizer as shown in FIG. 1. The oxidizer includes two opposed heat exchangers with a central combustion zone on communication with each. Each heat exchanger is composed of a matrix of stacked monolith blocks having a plurality of usually parallel vapor flow passages for process gas flow. The stacked blocks are appropriately aligned so that their vapor flow passages line up, defining a plurality of passageways from the heat exchanger column inlet to the outlet. However, such an arrangement can have a problem with high drag forces on the media blocks at the hot end of the heat exchange matrix (i.e., the end farthest from the inlet (or outlet, as the case may be) and closest to the combustion zone.
For a combination of high temperature and high air velocity, the drag force can be large enough to cause a media block to move out of the matrix, especially if the block has a slight downward tilt from the horizontal. These forces may cause cracking and premature failure of the monoliths resulting in costly, unscheduled downtime of the RTO and replacement of the monoliths, or at a minimum, downtime necessary to properly arrange the now altered matrix. This problem does not occur in heat exchangers wherein the matrix is vertically arranged (i.e., the gas flow is vertical through the matrix), since the gravitational forces due to the weight of the matrix resist such forces.
This problem in horizontal beds might be addressed by binding the matrix together such as with mortar, and by using expanding material to produce a compressive force on the matrix and hold it in place. Over time, however, both of these methods may fail, especially in view of the high temperature environment of the heat exchanger beds. Indeed, the mortar may breakdown or crack and the compressive force induced by the expanding material may relax.
It is therefore an object of the present invention to provide an arrangement of heat exchange media which allows for the horizontal or substantially horizontal flow of process gas from inlet to outlet, but counters the drag forces which may occur.